Orthogonal Frequency Division Multiplexing (OFDM) communication system has been adopted as the regional standards for the terrestrial television in Europe (DVB-SI) and Japan (ISDB-T). For DVB-SI for Europe this is described in “Digital Broadcasting Systems for Television, Sound, and Data Services; Framing Structure, Channel Coding and Modulation for Digital Terrestrial Television,” ETS 300 744,1997. For ISDB-T for Japan the reference is “Technical Specification of Terrestrial Digital TV Broadcasting,” ARIB draft specification, Ver. 0.5, December 2000.
OFDM's spread spectrum technique distributes the data over a large number of sub-carriers that are spread apart at precise frequencies. The spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing other frequencies than their own. The OFDM technique relies on the orthogonality properties of the fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT) to eliminate interference between carriers. At the transmitter, the precise setting of the carrier frequencies is performed by the IFFT. The data is encoded into constellation points by multiple (one for each carrier) constellation encoders. The complex values of the constellation encoder outputs are the inputs to the IFFT. For wireless transmission, the outputs of the IFFT are converted to an analog waveform, upconverted to a radio frequency, amplified, and transmitted. At the receiver, the reverse process is performed. The received signal is amplified, down converted to a band suitable for analog to digital conversion, digitized, and processed by a FFT to recover the carriers. The multiple carriers are then demodulated in multiple constellation decoders (one for each carrier), recovering the original data. Since an IFFT is used to combine the carriers at the transmitter and a corresponding FFT is used to separate the carriers at the receiver, the process has potentially zero intercarrier interference.
In wireless transmissions, the signal can be reflected or scattered from buildings, vehicles, trees, vegetation, and terrain features. Multiple copies of the signal, each with a different time delay that depends upon the path traveled, are summed at the receive antenna. This phenomenon is referred herein as Multi-path transmission, or more simply multi-path. Multi-path causes fading and attenuation in the frequency band, which if uncompensated causes unacceptably large number of errors in the decoding process. Multi-path interference, called the ghost phenomenon frequency encountered in the vehicular reception of the conventional terrestrial analog TV, should be much eliminated by this OFDM digital modulation and the introduction of the protective guard interval (GI) in the transmission.
Further, to avoid this, carriers with known amplitude and phase are transmitted for the purpose of measuring the wireless transmission channel. These carriers are known as training tones or pilot tones. These pilot tones are inserted periodically in the frequency and time domain in the transmitted OFDM symbol. Since the training tones are known apriori, the response of the wireless channel at the data tone frequencies can be interpolated from known responses at the training tone frequencies. The measured and interpolated channel responses, known as channel estimates, are used in the decoding process.
Conventionally the channel estimate for equalizing the data part of the received OFDM symbol has been achieved by this interpolating of the channel estimate obtained at the pilot signals. The following references discuss conventional channel estimate for equalizing.    1. Yagi, et al, “Development of OFDM Transmission Equipment for Digital Terrestrial Television Broadcasting,” ITE Technical Report Vol. 23, No. 34, May 1999.    2. Yamamoto, et al, “A study on Equalization technique for an OFDM System,” IEICE Technical Report IE98-91, November 1998.    3. Kisoda, et al. “Development of the OFDM Modem,” ITE Technical Report Vol. 21, No.44, August 1997.    4. Shima, et al, “A Study on Improving Characteristics of OFDM Equalizer Using Array Antennas,” ITE Annual Convention Mar. 1, 2000.    5. Zhao and Huang, “A Novel Channel Estimation Method of OFDM Mobile Communication Systems Based on Pilot Signals and Transform-Domain Processing,” Proceedings VTC 1997, pp 2089–2093.    6. Hossam H'Mimy, “Channel Estimation Based on Coded Pilot for OFDM,” Proceedings VTC 1997, pp. 1375–1379.    7. Multi-Carrier Spread Spectrum, pp. 169–178, Kluwar Academic Press. 1997.    8. Frescura, et al, “DSP-based OFDM Demodulator and Equalizer for Professional DVB-T Receivers,” IEEE Trans. Broadcasting, Vol. 45, No. 3, September 1999.
However, when the signal to noise ratio, called carrier to noise ratio in the following drops near to 0 dB, for instance in the shadow of the building or indoors, a good channel estimate becomes difficult due to the disruption of the pilot signals themselves.